mTORC1-dependent protein synthesis and autophagy uncouple in the regulation of Apolipoprotein A-I expression.
Animals
Apolipoprotein A-I
/ biosynthesis
Autophagy
/ genetics
Cholesterol
/ metabolism
Diet, High-Fat
Gene Expression Regulation
Hep G2 Cells
Hepatocytes
/ metabolism
Humans
Lipid Metabolism
Mechanistic Target of Rapamycin Complex 1
/ antagonists & inhibitors
Mice
Non-alcoholic Fatty Liver Disease
/ genetics
Nutritional Status
Proteasome Endopeptidase Complex
/ genetics
ApoA-I
Autophagy
mTORC1
Journal
Metabolism: clinical and experimental
ISSN: 1532-8600
Titre abrégé: Metabolism
Pays: United States
ID NLM: 0375267
Informations de publication
Date de publication:
04 2020
04 2020
Historique:
received:
18
12
2019
revised:
10
02
2020
accepted:
16
02
2020
pubmed:
23
2
2020
medline:
28
4
2020
entrez:
22
2
2020
Statut:
ppublish
Résumé
Apolipoprotein A-I (ApoA-I) is involved in reverse cholesterol transport as a major component of HDL, but also conveys anti-thrombotic, anti-oxidative, anti-inflammatory and immune-regulatory properties that are pertinent to its protective roles in cardiovascular, inflammatory and malignant pathologies. Despite the pleiotropy in ApoA-I functions, the regulation of intracellular ApoA-I levels remains poorly explored. HepG2 hepatoma cells and primary mouse hepatocytes were used as in vitro models to study the impact of genetic and chemical inhibitors of autophagy and the proteasome on ApoA-I by immunoblot, immunofluorescence and electron microscopy. Different growth conditions were implemented in conjunction with mTORC inhibitors to model the influence of nutrient scarcity versus sufficiency on ApoA-I regulation. Hepatic ApoA-I expression was also evaluated in high fat diet-fed mice displaying blockade in autophagy. Under nutrient-rich conditions, basal ApoA-I levels in liver cells are sustained by the balancing act of autophagy and of mTORC1-dependent de novo protein synthesis. ApoA-I proteolysis occurs through a canonical autophagic pathway involving Beclin1 and ULK1 and the receptor protein p62/SQSTM1 that targets ApoA-I to autophagosomes. However, upon aminoacid insufficiency, suppression of ApoA-I synthesis prevails, rendering mTORC1 inactivation dispensable for autophagy-mediated ApoA-I proteolysis. These data underscore the major contribution of post-transcriptional mechanisms to ApoA-I levels which differentially involve mTORC1-dependent signaling to protein synthesis and autophagy, depending on nutrient availability. Given the established role of ApoA-I in HDL-mediated reverse cholesterol transport, this mode of ApoA-I regulation may reflect a hepatocellular response to the organismal requirement for maintenance of cholesterol and lipid reserves under conditions of nutrient scarcity.
Sections du résumé
BACKGROUND
Apolipoprotein A-I (ApoA-I) is involved in reverse cholesterol transport as a major component of HDL, but also conveys anti-thrombotic, anti-oxidative, anti-inflammatory and immune-regulatory properties that are pertinent to its protective roles in cardiovascular, inflammatory and malignant pathologies. Despite the pleiotropy in ApoA-I functions, the regulation of intracellular ApoA-I levels remains poorly explored.
METHODS
HepG2 hepatoma cells and primary mouse hepatocytes were used as in vitro models to study the impact of genetic and chemical inhibitors of autophagy and the proteasome on ApoA-I by immunoblot, immunofluorescence and electron microscopy. Different growth conditions were implemented in conjunction with mTORC inhibitors to model the influence of nutrient scarcity versus sufficiency on ApoA-I regulation. Hepatic ApoA-I expression was also evaluated in high fat diet-fed mice displaying blockade in autophagy.
RESULTS
Under nutrient-rich conditions, basal ApoA-I levels in liver cells are sustained by the balancing act of autophagy and of mTORC1-dependent de novo protein synthesis. ApoA-I proteolysis occurs through a canonical autophagic pathway involving Beclin1 and ULK1 and the receptor protein p62/SQSTM1 that targets ApoA-I to autophagosomes. However, upon aminoacid insufficiency, suppression of ApoA-I synthesis prevails, rendering mTORC1 inactivation dispensable for autophagy-mediated ApoA-I proteolysis.
CONCLUSION
These data underscore the major contribution of post-transcriptional mechanisms to ApoA-I levels which differentially involve mTORC1-dependent signaling to protein synthesis and autophagy, depending on nutrient availability. Given the established role of ApoA-I in HDL-mediated reverse cholesterol transport, this mode of ApoA-I regulation may reflect a hepatocellular response to the organismal requirement for maintenance of cholesterol and lipid reserves under conditions of nutrient scarcity.
Identifiants
pubmed: 32084429
pii: S0026-0495(20)30050-0
doi: 10.1016/j.metabol.2020.154186
pii:
doi:
Substances chimiques
APOA1 protein, human
0
Apoa1 protein, mouse
0
Apolipoprotein A-I
0
Cholesterol
97C5T2UQ7J
Mechanistic Target of Rapamycin Complex 1
EC 2.7.11.1
Proteasome Endopeptidase Complex
EC 3.4.25.1
Types de publication
Journal Article
Research Support, Non-U.S. Gov't
Langues
eng
Sous-ensembles de citation
IM
Pagination
154186Informations de copyright
Copyright © 2020 Elsevier Inc. All rights reserved.
Déclaration de conflit d'intérêts
Declaration of competing interest None to declare.